在F#中实现ThreadStatic单例

Question

我正在学习F#，并且想实现ThreadStatic singleton。我使用的是我在一个类似的问题中发现的：F# How to implement Singleton Pattern (syntax)

下面的代码编译器抱怨说The type 'MySingleton' does not have 'null' as a proper value。

type MySingleton = 
    private new () = {}
    [<ThreadStatic>] [<DefaultValue>] static val mutable private instance:MySingleton
    static member Instance =
        match MySingleton.instance with
        | null -> MySingleton.instance <- new MySingleton()
        | _ -> ()
        MySingleton.instance

在此场景下，如何初始化实例？

Answer 1

我认为[<ThreadStatic>]会导致相当笨重的代码，尤其是在F#中。有一些方法可以更简洁地实现这一点，例如，使用ThreadLocal

open System.Threading

type MySingleton private () = 
  static let instance = new ThreadLocal<_>(fun () -> MySingleton())
  static member Instance = instance.Value

Answer 2

另一种F#y解决方案是将实例存储为option

type MySingleton = 
    private new () = {}

    [<ThreadStatic>; <DefaultValue>]
    static val mutable private instance:Option<MySingleton>

    static member Instance =
        match MySingleton.instance with
        | None -> MySingleton.instance <- Some(new MySingleton())
        | _ -> ()

        MySingleton.instance.Value

Answer 3

接近拉蒙所说的，将AllowNullLiteral属性应用于类型(默认情况下，在F#中声明的类型不允许'null‘作为正确值)：

[<AllowNullLiteral>]
type MySingleton = 
    private new () = {}
    [<ThreadStatic>] [<DefaultValue>] static val mutable private instance:MySingleton
    static member Instance =
        match MySingleton.instance with
        | null -> MySingleton.instance <- new MySingleton()
        | _ -> ()
        MySingleton.instance